The invention relates generally to distillation systems and particularly to distillation systems that include a reverse osmosis unit.
One of the maintenance problems that besets anyone operating a water still is the formation of scale in the evaporator. This can be reduced or eliminated by the removal or partial removal of the dissolved solids before the water is fed to the still. Ion exchange and reverse osmosis are some of the processes used in accomplishing the desired reduction in total solids.
There are drawbacks to the use of the reverse osmosis process. One of them is that reverse osmosis membranes have been quite expensive and have had a flux (which is the ability of the water to pass through the membrane) of such a capacity that a large amount of membrane has been necessary to produce a reasonable flow of water even when the water is pumped to the membrane at pressures up to 400 psig. Recently there have been improvements in membrane design which have made it possible to operate membranes at pressures as low as 50 psig. This means that a membrane of reasonable cost will produce approximately one gallon of water per hour when operated at a pressure of approximately 50 psig. Theoretically, therefore, one of such membranes could be placed ahead of a one gallon per hour still, and prevent scale formation in that still at a reasonable cost.
The reverse osmosis membranes have other shortcomings, however. They include the possibility of precipitation of such salts as calcium sulfate and calcium carbonate on the membrane surface unless some precautions are taken to prevent this precipitation. Also, if calcium carbonate precipitates on a cellulose acetate membrane the high pH created by the calcium carbonate will degrade and essentially destroy the membrane.
Methods to prevent this precipitation include softening the water, controlling the inlet pH with an acid feed system, or bypassing large quantities of water by the membrane surface to prevent the concentration of salts on the surface which can lead to the precipitation process. If the final method is employed, the production of water from the reverse osmosis unit may be only between 5% and 25% of the volume of the feed water. This ratio of product recovery to feed may not be significant for a one gallon per hour unit, but it does become significant as the production requirements increase to the range of 100 gallons per hour. Under these conditions the operator generally uses an acid feed system or perhaps pretreatment with a softener to save on feed water although these methods require additional equipment and materials.
It is an object of the invention to provide a water distillation system that can use the membrane surface bypass method with a reduction in the amount of feed water usually associated with that method. It is another object to make efficient use of RO unit waste water in a distillation system. Another object is to provide a system for heating RO unit water input without additional equipment. It is still a further object of the invention to provide an efficient, energy saving, and inexpensive liquid distillation system.